Formspec: change tabs with ctrl(+shift)+tab (#16167)

This change makes it easier to go to the next/previous tab using keyboard controls.

src/gui/guiFormSpecMenu.cpp

@@ -4068,6 +4068,31 @@ bool GUIFormSpecMenu::OnEvent(const SEvent& event)
 {
 	if (event.EventType==EET_KEY_INPUT_EVENT) {
 		KeyPress kp(event.KeyInput);
+		// Ctrl (+ Shift) + Tab: Select the (previous or) next tab of a TabControl instance.
+		bool shift = event.KeyInput.Shift;
+		bool ctrl = event.KeyInput.Control;
+		if (event.KeyInput.PressedDown && (event.KeyInput.Key == KEY_TAB && ctrl)) {
+			// Try to find a tab control among our elements
+			for (const FieldSpec &s : m_fields) {
+				if (s.ftype != f_TabHeader)
+					continue;
+
+				IGUIElement *element = getElementFromId(s.fid, true);
+				if (!element || element->getType() != gui::EGUIET_TAB_CONTROL)
+					continue;
+
+				gui::IGUITabControl *tabs = static_cast<gui::IGUITabControl *>(element);
+				s32 num_tabs = tabs->getTabCount();
+				if (num_tabs <= 1)
+					continue;
+
+				s32 active = tabs->getActiveTab();
+				// Shift: Previous tab, No shift: Next tab
+				active = (active + (shift ? -1 : 1) + num_tabs) % num_tabs;
+				tabs->setActiveTab(active);
+				return true; // handled
+			}
+		}
 		if (event.KeyInput.PressedDown && (
 				(kp == EscapeKey) ||
 				((m_client != NULL) && (kp == getKeySetting("keymap_inventory"))))) {

src/gui/guiTable.cpp

@@ -831,8 +831,10 @@ bool GUITable::OnEvent(const SEvent &event)
 			return true;
 		}
 		else if (event.KeyInput.Key == KEY_ESCAPE ||
-				event.KeyInput.Key == KEY_SPACE) {
+				event.KeyInput.Key == KEY_SPACE ||
+				(event.KeyInput.Key == KEY_TAB && event.KeyInput.Control)) {
 			// pass to parent
+			return IGUIElement::OnEvent(event);
 		}
 		else if (event.KeyInput.PressedDown && event.KeyInput.Char) {
 			// change selection based on text as it is typed